This invention relates to a method of making a nickel electrode particularly, but not exclusively, for use in secondary electrochemical cells (including nickel/cadmium,nickel/hydrogen, nickel/iron and nickel/zinc cells).
Existing nickel electrodes can in general be divided into three types:
pocket electrodes, PA1 sintered electrodes, and PA1 pressed electrodes. PA1 (a) contacting nickel hydroxide powder with a solution containing cobalt ions and precipitating a cobalt compound from said solution onto the nickel hydroxide powder, PA1 (b) before or after step (a), mixing the nickel hydroxide powder with a powdered conductive diluent and a powdered binder and pressing the resultant mixture into physical and electrical contact with a current collector.
In a pocket electrode, the active material of the electrode is contained in a thick, perforated pocket formed of nickel-plated steel. The resulting electrode is highly stable and exhibits a very long cycle life, typically of the order of thousands of cycles. However, this type of electrode suffers from the problem of being relatively heavy and hence its gravimetric energy density tends to be poor, typically varying between 0.06 and 0.09 Ah/gm of active material. In an attempt to overcome the problems experienced with pocket electrodes, sintered nickel electrodes were developed in which nickel powder is sintered at a temperature in the order of 900.degree.-1000.degree. C. and then is impregnated, either by a chemical or an electrochemical process, with the required active material. In view of the advantageous current carrying capacities of the sintered substrate, sintered nickel electrodes exhibit excellent power performance and improved gravimentric and volumetric energy density as compared with pocket nickel electrodes. Typically, the gravimetric energy density of a sintered electrode is of the order of 0.11-0.12 Ah/gm of active material, whereas the volumetric energy density is of the order of 0.35-0.4 Ah/c.c. of active material. Sintered electrodes also exhibit similar cycle lives to pocket electrodes, but suffer from the disadvantage that the production and material costs are high. Pressed electrodes are produced by pressing the active material into contact with a current collector and have the advantage that they are both lighter in weight than pocket electrodes and cheaper to produce than sintered electrodes. However, up to now, pressed electrodes have had a relatively short cycle life, typically of the order of several hundred cycles, so that they have not as yet proved viable for large scale production.
In each of the above types of nickel electrode, the active material normally comprises nickel hydroxide as the main constituent, which undergoes the following reversible reaction during discharge and subsequent charging: EQU Ni(OH).sub.2 +OH.sup..crclbar. .revreaction.NiOOH+H.sub.2 O+e.sup..crclbar.
The active material may also comprise a binder, such as polytetrafluoroethylene, and a conductive diluent such as graphite or nickel powder. Moreover, in order to improve the performance of the electrode, it is normal to incorporate a cobalt compound in the active material. Typically the cobalt compound is in the form of cobalt hydroxide and is present in an amount such that the active material contains between 5 and 8% of cobalt hydroxide by weight of the total weight of nickel hydroxide and cobalt hydroxide. Using this level of cobalt addition, it is readily possible to obtain sintered electrodes having a percentage utilization of the nickel hydroxide as high as 120-130%, although with pressed electrodes, the percentage utilization is normally only 70-100%. However, the use of such high cobalt additions in pressed electrodes suffers from the problem that it tends to cause swelling of the electrode so that, unless further additives (for example, cadmium hydroxide) are introduced into the active material, the service life of the electrode is reduced. In addition, with pressed and sintered nickel electrodes containing substantial quantities of cobalt hydroxide, it is found that the electrodes only slowly develop their capacity during charging and discharging cycles so that typically the full capacity is not reached until after 20-30 cycles.
In order to incorporate a cobalt compound in the active material of a nickel electrode, a number of methods have to date been proposed. The most commonly used is co-precipitation in which sodium or potassium hydroxide is added to a common solution containing cobalt and nickel ions so as to co-precipitate cobalt hydroxide and nickel hydroxide. The mixed hydroxide precipitate is then used as the active material of the electrode. An alternative method is to produce the cobalt and nickel hydroxide separately and then to physically blend them during production of the active material. In addition, it is known in the case of sintered and pocket electrodes to introduce the cobalt compound into the active material by contacting already formed nickel hydroxide with a solution of a cobalt salt and then precipitating the cobalt compound from this solution by the addition of an alkali metal hydroxide or carbonate to the solution. However, no relative advantages over the co-precipitation route have been gained and to date this method has not been employed in the production of pressed electrodes.
Following research which has now been conducted into alleviating the problems experienced with conventional nickel electrodes, it has been found that a pressed nickel electrode having improved performance can be obtained by ensuring that the active material contains a cobalt compound produced by precipitation from a cobalt containing solution in contact with already formed nickel hydroxide. Advantageous results are obtained with cobalt additions equal to or less than 3% of the total weight of the nickel hydroxide and cobalt compound and hence, in view of the high cost of cobalt, in practice the upper limit of the cobalt addition will be 3% or less of the total weight of the cobalt compound and nickel hydroxide. The resultant electrode is found to have an improved gravimetric energy density as high as 0.2 Ah/g, an excellent high rate performance, efficiency and cycle life, and to reach its full capacity after a small number of discharging and charging cycles. The percentage utilization of the nickel hydroxide is found to be as high as 130%.